Based on two different ways-the fluid energy equation and the fluid compressibility, the lumped parameter equations of Reactor Coolant System (RCS) depressurization during small break LOCA (SBLOCA) are derived and the two different derivations could verify each other by giving the identical result. And then, the transient equation is derived from the steady equation and is solved numerically. By comparing the results of steady and transient equations with that of NOTRUMP, it proves the equation’s rationality and validity. These lumped parameter equations could give the quantitative insight into the influence of the venting of water or steam through the Automatic Depressurization System (ADS) valves/break during LOCA depressurization phase.
During the SBLOCA depressurization phase of AP1000, the ADS4 valves are the most important venting paths. There are four ADS4 valves, which are mounted respectively on two sides of hot legs. Westinghouse originally assumed that the most limiting single failure was one of ADS4-B valves failure to open. But in truth, the most limiting failure should be one of ADS4-A valves failure to open, which has been validated by SNERDI with NOTRUMP calculations. However, it’s not easy to analyze the reason until it’s recognized that the key factor is the venting of steam, which is the theoretic conclusion above. This paper will give some insight into the limiting single failure criterion of SBLOCA, based on the theoretic conclusion above and the sensitivity analysis of NOTRUMP calculations.
